It is known that poly(ethylene glycol) can be added to polyoxymethylene to lower melt viscosity (Jap. Pat. No. 8816/'62). This is done in order to avoid high processing temperatures that might induce decomposition. However, this technique has been of limited value because commercially available poly(ethylene glycol) contains impurities and when used with well-known polyoxymethylene stabilizers, such as those disclosed in Alsup and Lindvig, U.S. Pat. No. 2,993,025, stabilization is inhibited.
The well-known stabilizers for polyoxymethylenes to improve thermal stability are superpolyamides of melting point less than 220.degree. C. The applicability of higher-softening polyamides for the purpose of thermally stabilizing polyoxymethylene is described in Johnson U.S. Pat. No. 4,098,843. The Johnson patent teaches that these higher-softening polyamides also reduce mold deposit. The higher-softening polyamides are predispersed in a suitable carrier resin in order to achieve small particle size when dispersed in the polyoxymethylene.
There is nothing in the art to suggest how to resolve the destabilizing effect of the addition of poly(ethylene glycol) to polyoxymethylene except via the use of increased amounts of polyamide stabilizers which have detrimental effects on mold and die deposit, surface appearance, and ultimately on physical properties. U.S. Pat. No. 4,098,843 shows no stabilizing advantage over U.S. Pat. No. 2,993,025 and, indeed, carries the implication of larger additive requirements for comparable stability because of the presence of the carrier resin.
It is desirable to obtain a polyoxymethylene molding blend which does not possess this destabilizing effect when poly(ethylene glycol) is present.